This invention relates to an optical telecommunications system. More specifically, this invention relates to an optical telecommunications system which uses multiple phase-compensated optical signals.
At present, digital and analog transmission employ a variety of systems for telecommunications including point-to-point microwave radio, optical fiber cable link, copper cable link, and communication satellite transmission. Such systems are used for transmitting telephone calls, television signals, and other audio and/or visual signals as well as various data telecommunications. In recent years, the trend has been towards the use of increasing numbers of optical fiber links. Such systems generally use optical fiber in a passive role for transmitting data and communications point-to-point using conventional electronics for all applications and multiplexing requirements. That is, the optical fiber cable between the transmitter and receiver is essentially a dumb link. The trend in recent years has been towards higher and higher data transmission rates reaching into the Gb/s range. This requires the use of more costly electronic components and optical sources. Much of the early single-mode fiber that has been deployed is unable to accommodate these high transmission rates. Furthermore, a variety of protocols are presently in use. These include DS3, SONET, International (E3), ATM, etc. When several of these are to be transmitted simultaneously over a common bus, it is necessary to digitally convert them to a single protocol.
In present systems, information is usually multiplexed in time-division format. The diverse signals are multiplexed together by combining them temporally. For example, 24 digital signal zero (DS0) level signals are sampled sequentially and combined to form the next level of signal transmission, which is T1 (DS1). The outputs of 4 T1 transmitters may be sampled and stacked sequentially in time by a T2 (DS2) multiplexer. Similarly, the outputs of 28 T1 or 7 T2 transmitters may be sequentially sampled and combined by a T3 (DS3) multiplexer. This process of combining or multiplexing lower level telemetry signals is repeated many times until signals in the GB/s range are produced.
The above approach has a number of disadvantages. European protocol differs from U.S. protocol. Thirty-two DS0 signals are combined by an E1 multiplexer, the European counterpart of T1. Thirty E1 channels transmit DS0 signals while the other two channels are used for signaling and alarm/supervision purposes. In general, European and U.S. standard telemetry is not mixed. The byte rates and formats differ. Likewise, while DS3 and synchronous optical network (SONET) formats may be combined in the same transmission facility, the DS3 is limited to non-add/drop[insert] applications. In other words, such arrangements make it difficult to drop out signals and insert other signals at intermediate ends of the transmission path. In such cases, at a point further down the facility, a portion of the signals are separated and diverted from the cable, while the remainder plus some additional information inserted at the same location continues to propagate along the cable. However, at such points, the multiplexed signal must be electronically broken down into basic DS3, DS2, DS1, DS0, ATM wideband, and fractional wide band data operating at DS3 and SONET rate, sorted, and recombined. This requires significant quantities of electronics including both a digital demultiplexer, one or more multiplexers and microprocessors as illustrated in the prior art FIG. 1.
Another disadvantage of the above approach is that narrow bandwidth single mode lasers are used as the optical source. Such narrow band sources are especially susceptible to back reflected light due to Brillioun scattering and/or other nonlinear optical effects. Backscattering places an upper limit on the intensity of the optical signal that can be propagated through the optical transmission line and the resulting lower optical intensity in turn requires that optical regenerators be spaced closer together than might be the case if an optical source having greater intensity could be used.
The amount of backscattered light is a direct function of the optical intensity and an inverse function of the optical bandwidth of the source. Thus, optical sources having narrow bandwidths experience more backscattered light than do optical sources of the same intensity but wider bandwidths
Yet another disadvantage of the above approach is that optical signals propagating through fiber-optic transmission lines undergo optical dispersion; that is, the propagation velocity in optical fiber is a function of wavelength. This leads to a broadening of transmitted light pulses as they propagate along the fiber. The broadening results in signal distortion and leads to intersymbol interference (ISI), and an increase in bit-error rate (BER), and/or a reduction in useable transmission bandwidth. The amount of dispersion is a direct function of the optical path length. Thus, optical dispersion leads to reduced spacing between optical regenerators.
In the case of SONET protocol, the payloads in the optical cable use standard bit rates of 51.84 Mb/s (optical carrier level 1 or OC1), 155.52 Mb/s (OC3), 622.08 Mb/s (OC12), 1.244 Gb/s (OC24), and 2.488 Gb/s (OC48) and above. The corresponding electronic interfaces are designated as synchronous transport level 1 (ANSI=STS-1), STS-3 equivalent to synchronous transport module level 1 (CCITT=STM-1), STS-12/STM-4, STS-24, and STS-48/STM-16, respectively and above. Much of the original single mode fiber deployed is unable to transport payloads in the Gb/s range.
U.S. Pat. No. 4,477,423, issued Oct. 16, 1984 to Edward F. Carome and one of the present inventors', Charles M. Davis, prior and hereby incorporated by reference, discloses a technique of using optical phase modulation to detect electric fields. An interferometer configuration is used.
U.S. Pat. No. 4,755,668, issued Jul. 5, 1988, to one of the present inventors', Charles M. Davis, prior and hereby incorporated by reference, discloses optical phase modulation interferometer techniques for use with a plurality of sensors where the optical signal corresponding to each sensor is separately and individually distinguished by a fixed optical phase difference. A separate and individual interferometer configuration is used for each sensor. These interferometers are distinguished from each other by the path length differences between the two arms of the interferometers.
U.S. Pat. No. 4,728,191, issued Mar. 1, 1988, to one of the present inventors', Clarence J. Zarobila, prior and hereby incorporated by reference, discloses the use of phase-compensation interferometry employing a 3.times.3 coupler/splitter.
The following patents disclose various other phase modulation techniques for use with telecommunications and/or sensors:
______________________________________ Patent No. Inventor ______________________________________ 4,699,513 Brooks et al 4,848,906 Layton 4,860,279 Falk et al 4,866,698 Huggins et al 4,822,775 Coleman 5,191,614 LeCong 5,223,967 Udd ______________________________________
Although the above and other techniques have been generally useful, they have often been subject to one or more disadvantages. For example, the capacity to carry a high bit rate payload within a single transmission path, such as optical fiber, is often limited. Generally, add/drop[/insert] operations (picking off a signal and inserting another signal) at an intermediate stage in a transmission path require complex electronics. Some techniques provide questionable security for transmission of telecommunications such as audio, video, and/or data. Some techniques do not readily or easily provide full duplex transmission within a single fiber. Some prior techniques do not readily allow redundant transmissions. Most techniques do not allow simultaneous transmission of the various different protocols. Most techniques do not allow simultaneous transmission of analog and digital data. Most techniques require complex, high speed digital electronics in order to transmit high Mb/s and Gb/s payloads.